Radiofrequency (RF) pulses applied during MR imaging causes potential heating of tissue due to the RF power or energy absorbed. The rate at which radiofrequency energy is deposited into the tissue is defined as the specific absorption rate (SAR) and is generally measured in units of watts per kilogram (W/kg). As field strength increases, the RF power deposition and the associated local specific absorption rate (SAR) also increase. For example, doubling the field strength from 1.5 Tesla (T) to 3 T results in a quadrupling of the SAR. Therefore, SAR limits are legally mandated to ensure patient safety. Hot spots, or regions of concentrated energy deposition, occur in tissue with high conductivity such as muscles, the spinal cord and the eyes, or at tissue interfaces with high dielectric constant such as muscle-fat or muscle-bone interfaces.
The SAR becomes a limiting factor for many MR imaging applications because it is dependent on field strength, RF power, flip angle, transmit coil type, and body size. In a multi-shot MR sequence, the same RF pulses are typically repeated numerous times in order to acquire an image. For example, some sequences call for applying the same RF excitation pulse before collection of each line of k-space data. Other RF pulses, such as preparation pulses, RF manipulation pulses, and the like, may also be applied multiple times during a sequence. It is possible to limit the magnitude of hot spots by limiting the amplitude and increasing the duration of the RF pulses; however, in certain imaging sequences the contrast is dependent on the flip angle of the RF excitation pulse and increased duration may yield motion artifacts.
In MR scanners with multiple, independently controlled transmit (Tx) elements, each pulse is the combination of the contributions of all of the elements. This provides greater flexibility in designing RF pulses. For example, electrical field information can be incorporated into the RF pulse design to minimize the SAR. Minimal SAR RF pulses can be selected from the large solution space due to the extra degree of freedom in the RF pulse design. For an N-channel Tx system and a small flip angle, the excitation pattern can be written in matrix notation as:m=Ab  (equation 1)where m describes a target excitation pattern, A is a system matrix, and b is a matrix of concatenated RF pulses bn (1≦n≦N) of the individual Tx elements. Provided that the RF field inside the subject responds linearly to the currents driving the field, the SAR can be expressed in a quadratic form in the pulse samples:SAR=b†Qb  (equation 2)where † denotes the conjugate transpose and Q is a block-diagonal positive definite matrix corresponding to a specific subject volume calculated from a solution of Maxwell's equations. In regimes with large flip angles, the excitation responds non-linearly to the RF field and equation 1 can be adapted for the non-linearity.
A proposed method for local SAR hot-spot reduction (Graesslin I, et al. [2008] ISMRM 16:621) incorporates knowledge about the spatial SAR distribution of an initial RF pulse into Q and a relaxed minimization of the equation b†Qb such that m=Ab. Using an initial RF pulse that is optimal with respect to global SAR, reduction of a single hot-spot was achieved via:Q=ΣiqiQregion(i)  (equation 3)where qi are real weighting factors specifying a trade-off between different hotspot regions. However, in Graesslin I, et al. [2008] ISMRM 16:621, the same RF pulses bn are repeatedly applied during a multi-shot imaging sequence creating a hotspot in the same place repetitively in which the cumulative effect may be undesirable.
The present application provides a new and improved method and apparatus which overcomes the above-referenced problems and others.